Electron beam apparatuses have many functions in a plurality of industrial fields, in particular in fields having a high demand for structuring and inspecting specimens within the micrometer and nanometer scale. Typical applications of electron beam apparatuses include inspection of semiconductor apparatuses during manufacturing, exposure systems for lithography, detecting apparatuses and testing systems. Electron beams offer superior spatial resolution compared to e.g., photon beams, due to their short wavelengths.
An important factor limiting the spatial resolution of electron beam apparatuses is the chromatic aberration of the electron optical system. The chromatic aberration can be reduced by reducing the energy spread of the electron beam. To reduce the energy spread, very expensive and complicated monochromators have been designed. These monochromators have the further disadvantage of reducing the beam intensity considerably.
When electron sources based on field emission are used, the energy spread is generally smaller than that of thermal electron sources. Such field emission electron sources are used e.g., in low voltage scanning electron microscopes (LVSEMs), wherein low voltage is generally defined as a voltage of less than 5 keV, in particular less than 1 keV. However, there remains the desire to further reduce the energy spread of the electron beam using simple and cost-effective means e.g., in order to further reduce chromatic aberrations.